I think it's a tricky business from a value proposition point of view. What advantage does it have over, say, Google Calendar?
I agree that its a bit too light weight to make into a business just yet as there are easy substitutions (GCalendar and Exchange offer similar functionality) But as a subscription service / API for another web property to hook into (a middleware layer if you will) it might get some traction with an e-vite or a Meetup type site.
I would really appreciate it!
What kind of advertising do you have in mind?
See below @26:53
http://www.livestream.com/500startups/video?clipId=pla_71863...